Fluorine-containing optical material and fluorine-containing copolymer

ABSTRACT

There is provided a fluorine-containing optical material and a fluorine-containing copolymer which have excellent heat resistance and flexibility in addition to optical characteristics, particularly a material suitable as a clad material for heat resistant plastic optical fiber. The fluorine-containing optical material comprises from 15 to 62% by mole of a structural unit (a) derived from hexafluoroneopentyl methacrylate, from 12 to 70% by mole of a structural unit (b) derived from methyl methacrylate and from 1 to 40% by mole of a structural unit (c) which is derived from a fluorine-containing monomer and is copolymerizable therewith, and is used as various optical materials, and is useful particularly as a clad material for heat resistant optical fiber.

TECHNICAL FIELD

The present invention relates to a fluorine-containing optical materialand a fluorine-containing copolymer which are excellent in heatresistance and flexibility in addition to optical characteristics. Theoptical material of the present invention is suitable particularly as aclad material for heat resistant plastic optical fiber.

BACKGROUND ART

Hitherto polycarbonate, non-crystalline polyolefin, acrylic resin andthe like have been studied as a plastic optical material, but there havebeen no materials having high heat resistance (high glass transitiontemperature Tg) and a low refractive index. For example, with respect topolycarbonate, heat resistance is high (Tg: 145° C.) but a refractiveindex is also high (1.58). It is just the same with non-crystallinepolyolefin (Tg: 171° C., refractive index: 1.51). With respect to anacrylic material, a refractive index thereof is decreased (not more than1.45) by using a fluorinated acrylate, but an obtained polymer isinsufficient in heat resistance (Tg<100° C.).

Recently particularly plastic optical fibers have been used as a cablefor LAN of cars. For such plastic optical fibers, polymethylmethacrylate (PMMA) is used as a core material therefor because ofexcellent transparency and a high refractive index, and a materialhaving a refractive index lower than that of PMMA is needed as a cladmaterial. In that case, as mentioned above, a material having a lowrefractive index has a problem with heat resistance.

Fluorine-containing copolymers obtained from perfluoro-t-butylmethacrylate (JP49-129545A), hexafluoroneopentyl methacrylate(JP1-149808A, JP2-110112A, JP2-1711A) or α-fluoroacrylate (JP61-118808A)are proposed as materials satisfying such properties mentioned above.

However, perfluoro-t-butyl methacrylate has a disadvantage thatflexibility of an obtained copolymer is lowered and adhesion to asubstrate (core material) is inferior. Also in the case ofα-fluoroacrylate, a preparation process is restricted and therefore,cost for preparing a copolymer becomes high and coloring arises due toheating, which are points to be improved.

Also with respect to a copolymer comprising hexafluoroneopentylmethacrylate (hereinafter referred to as 6FNPM as case demands) as acopolymerizable component, a high Tg and a low refractive index are notexhibited in all ranges of polymer composition disclosed in thementioned patent publications. Particularly when the proportion of 6FNPMis high, there is a case where the copolymer becomes fragile as anoptical material and flexibility becomes insufficient. On the otherhand, when the proportion of 6FNPM is low, effects of making Tg high andrefractive index low which are inherent characteristics of 6FNPM are notobtained.

For example, JP1-149808A discloses an optical material obtained from acopolymer comprising 6FNPM and methyl methacrylate (MMA) in a weightratio of 50/50 (27/73 in mole ratio), but this material is comparativelyhigh in a refractive index. Also JP2-110112A discloses an opticalmaterial obtained from a copolymer comprising 6FNPM and MMA in a weightratio of 90/10 (77/23 in mole ratio), but this material is inferior inflexibility. Further JP2-1711A discloses only a copolymer comprising6FNPM in an amount of up to 20% by mole.

DISCLOSURE OF INVENTION

The present inventors have made intensive studies and have found that afluorine-containing copolymer obtained in combination of 6FNPM or acompound analogous thereto, MMA and as case demands, otherfluorine-containing monomer in a specific composition exhibits physicalproperties very useful as an optical material, and have completed thepresent invention. Those specific requisites are not disclosed andtaught in prior art documents.

The present invention relates to an optical material (hereinafterreferred to as “optical material 1”) which comprises afluorine-containing copolymer comprising from 32 to 36% by mole of astructural unit (a) represented by the formula (1):

wherein X¹ is H, CH₃, F, CF₃ or C1; Rf² and Rf² are the same ordifferent and each is a perfluoroalkyl group having 1 to 5 carbon atoms;R¹ is a hydrocarbon group having 1 to 5 carbon atoms which may besubstituted with fluorine atom, and from 64 to 68% by mole of astructural unit (b) derived from methyl methacrylate.

Also the present invention relates to a fluorine-containing opticalmaterial (hereinafter referred to as “optical material 2”) whichcomprises a fluorine-containing copolymer comprising from 15 to 62% bymole of the structural unit (a) represented by the above-mentionedformula (1), from 12 to 70% by mole of a structural unit (b) derivedfrom methyl methacrylate and from 1 to 40% by mole of a structural unit(c) (excluding the structural unit (a)) derived from afluorine-containing monomer which is copolymerizable therewith.

The fluorine-containing copolymer used for the optical material 2 ispreferably a fluorine-containing copolymer comprising from 23 to 50% bymole of the structural unit (a), from 33 to 70% by mole of thestructural unit (b) and from 1 to 40% by mole of the structural unit(c).

The fluorine-containing copolymer which has a weight average molecularweight of from 10,000 to 1,000,000 and comprises from 32 to 36% by moleof the structural unit (a) represented by the formula (1) and from 64 to68% by mole of the structural unit (b) derived from methyl methacrylate,and a fluorine-containing copolymer which has a weight average molecularweight of from 10,000 to 1,000,000 and comprises from 15 to 62% by moleof the structural unit (a) represented by the formula (1), from 12 to70% by mole of the structural unit (b) derived from methyl methacrylateand from 1 to 40% by mole of a structural unit (c2) represented by theformula (2a):

wherein X³ is H, CH₃, F, CF₃ or Cl; R³ is H or a fluoroalkyl group; thestructural unit represented by the formula (1) is excluded, and when R³is H, X³ is neither H nor CH₃,which can be used for the optical materials 1 and 2 of the presentinvention, respectively are novel fluorine-containing copolymers.

BEST MODE FOR CARRYING OUT THE INVENTION

The optical material 1 of the present invention comprises the copolymercomprising from 32 to 36% by mole of the structural unit (a) representedby the formula (1) and derived from a fluoroacrylate derivative and from64 to 68% by mole of the structural unit (b) derived from methylmethacrylate (MMA).

In the fluoroacrylate derivative (1), it is preferable that X¹ is H,CH₃, F, CF₃ or Cl, particularly preferably CH₃ or F, further preferablyCH₃; Rf¹ and Rf² are the same or different and each is a perfluoroalkylgroup having 1 to 5 carbon atoms, concretely CF₃, CF₂CF₃, CF₂CF₂CF₃,CF₂CF₂CF₂CF₃ or CF₂CF₂CF₂CF₂CF₃, particularly preferably CF₃; R¹ is ahydrocarbon group having 1 to 5 carbon atoms which may be substitutedwith fluorine atom, concretely CH₃, CH₂CH₃, CH₂CH₂CH₃, CH₂CH₂CH₂CH₃,CH₂CH₂CH₂CH₂CH₃, CH₂CF₃, CH₂CH₂CF₃ or CH₂CF₂CF₃, particularly preferablyCH₃.

Non-limiting examples of the compound introducing the structural unit(a) are hexafluoroneopentyl methacrylate (6FNPM: X¹=CH₃, Rf¹=Rf²=CF₃,R¹=CH₃), hexafluoroneopentyl α-fluoroacrylate (6FNPF: X¹=F, Rf¹=Rf²=CF₃,R¹=CH₃), 2,2-bistrifluoromethylbutyl methacrylate (X¹=CH₃, Rf¹=Rf²=CF₃,R¹=CH₂CH₃), 2,2-bistrifluoromethylbutyl α-fluoroacrylate (X¹=F,Rf¹=Rf²=CF₃, R¹=CH₂CH₃),

wherein X¹=CH₃, Rf¹=CF₃, Rf²=CF₃, R¹=CH₂CF₃and the like. Among them, 6FNPM and 6FNPF are preferred from theviewpoint of excellent heat resistance and easy synthesis, and 6FNPM isparticularly preferred.

If the amount of the structural unit (a) is less than 32% by mole, arefractive index is increased, for example, like the copolymer in aweight ratio of 50/50 (namely, 27/73 in mole ratio) disclosed inJP1-149808A, and an effect of the present invention cannot be exhibitedsufficiently. A high Tg, a low refractive index and excellentflexibility can be obtained within such a very narrow range of an amountof the structural unit (a). If the amount of the structural unit (a)exceeds 36% by mole, the optical material 1 becomes hard and fragile,and is difficult to be applied to products requiring flexibility such asa clad material for optical fibers. A lower limit of the amount of thestructural unit (a) is preferably 33% by mole, and a preferred upperlimit thereof is 35% by mole.

The optical material 2 of the present invention comprises thefluorine-containing copolymer containing three or more monomers whichcomprises the structural unit (a) and the structural unit (b) asessential components and further the structural unit (c) derived from afluorine-containing monomer and as case demands, a structural unit (d)as an optional component derived from a copolymerizable monomer otherthan the monomers for the structural units (a) and (c).

In the optical material 2, if the amount of the structural unit (a) isdecreased, Tg decreases and a refractive index increases, and both of ahigh Tg (heat resistance) and a low refractive index cannot be satisfiedat the same time. A lower limit of the amount of the structural unit (a)is preferably 20% by mole, particularly preferably 28% by mole. As theamount of the structural unit (a) is increased, mechanical propertiessuch as flexibility become unsuitable for the use as an opticalmaterial. An upper limit thereof is 60% by mole, preferably 50% by mole.

Also if the amount of the structural unit (b) is decreased, mechanicalproperties such as flexibility become unsuitable for the use as anoptical material. A lower limit of the amount of the structural unit (b)is preferably 20% by mole, particularly preferably 33% by mole. As theamount of the structural unit (b) is increased, the fluorine-containingstructural unit is decreased, and both of a high Tg (heat resistance)and a low refractive index cannot be satisfied at the same time. Anupper limit thereof is preferably 70% by mole, particularly preferably68% by mole.

The structural unit (c) makes a refractive index low and Tg high due tofluorine atoms contained therein, and also contributes to enhanceflexibility. Therefore, when the amount of the structural unit (a) issmall, the amount of the structural unit (c) is increased, and when theamount of the structural unit (a) is large, the amount of the structuralunit (c) is adjusted within a range of from 1 to 40% by mole to enhanceflexibility while maintaining the balance with the amount of thestructural unit (b).

Usually in order to obtain a copolymer having a composition in anintended mole ratio, monomers having the respective parts by weightcorresponding to mole ratios may be polymerized. For example, in thecase of the optical material 2 of the present invention, when themolecular weights of each monomer providing the structural units (a),(b) and (c) are assumed to be M1, M2 and M3, respectively and the moleratios thereof are assumed to be m1, m2 and m3, respectively, a weightfraction of the monomer providing the structural unit (a) is representedby (m1×M1)/(m1×M1+m2×M2+m3×M3), a weight fraction of the monomerproviding the structural unit (b) is represented by(m2×M2)/(m1×M1+m2×M2+m3×M3), and a weight fraction of the monomerproviding the structural unit (c) is represented by(m3×M3)/(m1×M1+m2×M2+m3×M3).

In the fluorine-containing copolymer used in the optical material 2, thepreferred structural unit (c) is a structural unit (c1) represented bythe formula (2):

wherein X² is H, CH₃, F, CF₃ or Cl; R² is H or a fluoroalkyl group; thestructural unit represented by the formula (1) is excluded, and when R²is H, X² is neither H nor CH₃,

When the structural unit (cl) is used as the structural unit (c), Tg anda refractive index can be adjusted more minutely.

X² is preferably H, CH₃, F, CF₃ or Cl, particularly preferably CH₃ or F,further preferably CH₃.

Examples of R² are, for instance, —CH₂CF₃, —CH₂CF₂CF₃, —CH(CF₃)₂,—CH₂(CF₂)₄F, —CH₂CH₂(CF₂)₄F, —CH₂CH₂(CF₂)₆F, —CH₂CH₂(CF₂)₈F,—CH₂CF₂CF₂H, —CH₂CF₂CF₂CF₂H, —CH₂CF₂CFHCF₃, —CH₂(CF₂)₄H, —CH₂(CF₂)₅H,—CH₂(CF₂)₆H, —CH₂(CF₂)₈H and the like.

Particularly preferred structural unit (c1) is the structural unit (c2)represented by the formula (2a):

wherein X³ and R³ are as defined supra.

Particularly when the number of carbon atoms of R² or R³ in thestructural unit (c1) or (c2), respectively is decreased, an effect ofimparting flexibility tends to be lowered, and in the case of a longchain, heat resistance tends to be lowered (Tg decreases). The number ofcarbon atoms is preferably from 4 to 6. Further it is preferable that anend of R² or R³ is H from the viewpoint of excellent compatibility withpolymethyl methacrylate (PMMA) and excellent solubility in a solvent.

Concretely R² or R³ is preferably a fluoroalkyl group represented by theformula (3):—CH₂C_(n)F_(2n)H   (3)wherein n is an integer of from 3 to 5, and is further preferably—CH₂C₄F₈H.

It is preferable that in the formulae (2) and (2a) representing thestructural units (c1) and (c2), respectively, X² and X³ are —CH₃ becauseuniformity of copolymerization with MMA of the structural unit (b) isexcellent.

Examples of the preferred monomer providing the structural unit (c) are,for instance,

and the like. Among them, 8FM, 6FM, 10FM and HFIPM are preferredparticularly because bending property, mechanical properties and heatresistance thereof are excellent.

In the present invention, a copolymer comprising the above-mentionedstructural units (a), (b) and (c) (c1 and c2) and the structural unit(d) derived from a non-fluorine-containing monomer (excluding methylmethacrylate) being copolymerizable with the monomers therefor can beused.

Examples of the copolymerizable non-fluorine-containing monomerproviding the structural unit (d) are acrylic acid (AA), methacrylicacid (MA), 2-hydroxyethyl methacrylate (2-HEMA), glycidyl methacrylate(GMA), ethyl acrylate (EA) and the like. Particularly acrylic acid andmethacrylic acid are preferred because property for increasingmechanical strength can be imparted to the copolymer. The content ofstructural unit (d) in the copolymer is from 0 to 10% by mole,preferably from 0 to 5% by mole, particularly preferably from 0 to 1% bymole. A lower limit of the content of structural unit (d) may be oneexhibiting an effect of copolymerization thereof, and is usually, forexample, about 0.01% by mole.

Non-limiting examples of the preferred fluorine-containing copolymer forthe optical material 2 are 6FNPM/MMA/8FM, 6FNPF/MMA/8FM,6FNPM/MMA/HFIPM, 6FNPM/MMA/ 10FM, 6FNPM/MMA/8FM/MA and the like.Particularly 6FNPM/MMA/8FM and 6FNPM/MMA/8FM/MA are preferred from theviewpoint of excellent bending property and heat resistance.

A weight average molecular weight of the fluorine-containing copolymerused for the optical materials 1 and 2 (hereinafter referred to simplyas “optical materials” unless discriminated from each other) of thepresent invention is preferably from 10,000 to 1,000,000, furtherpreferably from 50,000 to 800,000, particularly preferably from 100,000to 500,000 because of good solubility in a solvent, a rather low meltviscosity and good moldability.

The optical materials of the present invention have a high Tg, a lowrefractive index and excellent flexibility, and preferred are thosehaving Tg of not less than 100° C., a refractive index of not more than1.440 and a fluorine content of not less than 20% by weight.

Tg is preferably not less than 105° C. The upper limit of Tg is usuallyup to 150° C., and the higher, the better. The refractive index ispreferably not more than 1.430. The lower limit of refractive index isup to 1.415, and the lower, the more preferable. The fluorine content ispreferably not less than 30% by weight, further preferably not less than35% by weight. The upper limit of fluorine content is determineddepending on the polymer composition, and is about 50% by weight.

With respect to Tg (glass transition temperature) defined in the presentinvention, by using DSC (differential scanning calorimeter), in the 1strun, temperature is raised up to 200° C. at a temperature elevating rateof 10° C./min, followed by maintaining at 200° C. for one minute andcooling down to 25° C. at a temperature decreasing rate of 10° C./min,and then a center point of heat absorption curve obtained in the 2nd runof heating at a temperature elevating rate of 10° C./min is assumed tobe Tg.

Also the refractive index is measured with an Abbe's refractometer at25° C. using sodium D line as light source.

The fluorine content (% by weight) is obtained by burning 10 mg ofsample by an oxygen flask combustion method, absorbing cracked gas in 20ml of de-ionized water and then measuring a fluorine ion concentrationin the fluorine ion-containing solution through fluoride-ion selectiveelectrode method (using a fluorine ion meter model 901 available fromOrion).

The optical materials of the present invention are, as mentioned supra,different from conventional optical materials from the viewpoint ofexcellent flexibility. Flexibility is important characteristic requiredin flexible devices, for example, optical fiber, optical interconnectionand flexible circuit.

For evaluation of flexibility, a fluorine-containing copolymer is heatedto 230° C. and extruded through an orifice to make 1 mm diametercopolymer fibers. One roll of the fiber is wound on round steel barshaving different radiuses in an environment of 25° C., and flexibilityis evaluated by a radius of the round bar when cracking occurs on thecopolymer fiber. In the present invention, round bars having a radius of6 mm, 10 mm, 15 mm, 20 mm and 30 mm are used.

Among fluorine-containing copolymers having a polymer composition beyondthe present invention, there are ones satisfying the requirements forrefractive index, but flexibility thereof is 15 mm (radius of round bar)or more. In the optical materials of the present invention, by furtheradjusting the polymer composition while the mentioned characteristicsare satisfied, there can be provided an optical material in whichcracking does not occur when wounding on a 10 mm (radius of round bar)and even when wounding on a 6 mm (radius of round bar).

The optical materials of the present invention also exhibit otherexcellent properties such as a thermal decomposition temperature Td,light transmittance and a melt index MI. Those properties are explainedin examples.

The optical materials of the present invention can be used as materialsfor various optical devices, for example, material for a clad of opticalfiber, antireflection coating material, lens material, optical waveguidematerial, prism material, optical window material, optical memory discmaterial, non-linear optical element material, hologram material,photorefractive material, material for sealing of light emitter, liquidcrystal panel material and the like.

The optical materials of the present invention are suitable as amaterial for clad for optical fiber, particularly as a material for cladfor optical fiber using a core made of polymethyl methacrylate (PMMA).

Optical fibers produced using PMMA as a material for core and theoptical materials of the present invention as a material for clad have ahigh Tg, excellent heat resistance and excellent flexibility in additionto characteristics as an optical fiber, and therefore, are useful as afiber to be provided in a narrow space under high temperatureenvironment. The optical materials of the present invention exhibit anexcellent effect which cannot be seen in other materials when provided,for example, in an engine room, instrument panel, roof, inside of headlamp and the like of car.

The present invention also relates to the fluorine-containing copolymerhaving a weight average molecular weight of from 10,000 to 1,000,000 andcomprising from 32 to 36% by mole of the structural unit (a) representedby the above-mentioned formula (1) and from 64 to 68% by mole of thestructural unit (b) derived from methyl methacrylate, and thefluorine-containing copolymer having a weight average molecular weightof from 10,000 to 1,000,000 and comprising from 15 to 62% by mole of thestructural unit (a) represented by the above-mentioned formula (1), from12 to 70% by mole of the structural unit (b) derived from methylmethacrylate and from 1 to 40% by mole of the structural unit (c2)represented by the above-mentioned formula (2a).

Preferred examples of those fluorine-containing copolymers are the sameas those raised supra in the explanation of the optical materials.

For preparation of the fluorine-containing copolymers of the presentinvention, there can be employed polymerization methods which are usedgenerally such as bulk polymerization method, solution polymerizationmethod, suspension polymerization method and emulsion polymerizationmethod.

In the bulk polymerization method, solution polymerization method andsuspension polymerization method, examples of a polymerization initiatorare, for instance, radical polymerization initiators such asazobisisobutyronitrile,1,1-bis(t-butylperoxy)3,3,5-trimethylcyclohexane, dialkyl peroxide,diacyl peroxide, peroxy ketal and di-t-butyl peroxide (Perbutyl D). Inthe emulsion polymerization method, there are used, for example,persulfates such as ammonium persulfate and potassium persulfate; orredox initiators comprising oxidizing agents such as the mentionedpersulfates, reducing agents such as sodium sulfite, and salts oftransition metals such as ferrous sulfate (II).

In the above-mentioned bulk polymerization method, solutionpolymerization method and suspension polymerization method, it ispreferable to use a chain transfer agent such as mercaptans foradjusting a molecular weight of the fluorine-containing copolymer.Examples of the chain transfer agent are, for instance, compounds havingmercapto group such as n-butyl mercaptane, lauryl mercaptane, n-octylmercaptane, n-butyl mercaptoacetate, isooctyl mercaptoacetate and methylmercaptoacetate.

In the above-mentioned solution polymerization method and suspensionpolymerization method, represented examples of a solvent arefluorine-containing solvents such as HCFC-225 and hydrocarbon solventssuch as butyl acetate and methyl isobutyl ketone.

A polymerization temperature is usually determined within a range offrom 0° C. to 100° C. in consideration of a decomposition temperature ofthe polymerization initiator, and in many cases, a polymerizationtemperature within a range of from 10° C. to 80° C. is preferablyemployed.

In the present invention, the weight average molecular weight in theabove-mentioned polymerization reaction can be adjusted usually within arange of from 10,000 to 1,000,000 (based on polystyrene according toGPC), preferably from 100,000 to 500,000.

Those fluorine-containing copolymers have the above-mentioned ranges ofTg, refractive index and fluorine content.

The present invention is then explained by means of examples, but is notlimited to them. In Examples and Comparative Examples, “part” means“part by weight”.

In the following Examples, physical properties are evaluated by usingthe following equipment and measuring conditions.

(1) NMR: AC-300 available from BRUKER

Measuring conditions of ¹H-NMR: 300 MHz (tetramethylsilane=0 ppm)

Measuring conditions of ¹⁹F-NMR: 300 MHz (trichlorofluoromethane=0 ppm)

(2) IR analysis: Measuring is carried out at room temperature with aFourier-transform infrared spectrophotometer 1760X available from PerkinElmer Co., Ltd.

(3) GPC: A number average molecular weight is calculated from the datameasured with gel permeation chromatography (GPC) by using GPC HLC-8020available from Toso Kabushiki Kaisha and columns available from Shodex(one GPC KF-80 1, one GPC KF-802 and two GPC KF-806M were connected inseries) and flowing tetrahydrofuran (THF) as a solvent at a flowing rateof 1 ml/min.

EXAMPLE 1

In a 500 ml glass flask, 50 parts of 6FNPM, 30 parts of methylmethacrylate (MMA), 20 parts of 8FM, 0.04 part of n-lauryl mercaptaneand 0.025 part of azoisobutyronitrile were dissolved and mixed, followedby repeating deaeration and replacement of the inside of the flask withnitrogen. Then after sealing of the flask, polymerization was carriedout at 70° C. for 16 hours.

After completion of the polymerization, 300 g of acetone was added tothe product to dissolve the product therein, and the obtained solutionwas poured into 5 liter of methanol. Then the precipitatedpolymerization product was separated from the solution, followed bydrying at 100° C. for 10 hours under reduced pressure to obtain 92 g(yield 92%) of solid polymer.

According to ¹H-NMR, ¹⁹F-NMR and IR analyses, the obtained polymer wasfound to be a copolymer comprising 6FNPM, MMA and 8FM in a percent bymole ratio of 34/54/12. The fluorine content thereof was 32% by weight.

Also the weight average molecular weight, refractive index, glasstransition temperature, thermal decomposition temperature, melt index,light transmission and flexibility of the obtained copolymer weredetermined. The results are shown in Table 1.

Those properties were measured by the following methods.

(1) Weight Average Molecular Weight (Mw)

Measured by GPC method (based on polystyrene)

(2) Refractive Index

Measured by the method mentioned supra (25° C.). A refractometer used isAbbe's refractometer available from Kabushiki Kaisha Atago Kogaku KikiSeisakusho.

(3) Glass Transition Temperature (Tg)

Measured by the method mentioned supra. A differential scanningcalorimeter used is one available from Seiko Denshi Kabushiki Kaisha.

(4) Thermal Decomposition Temperature (Td)

A thermal decomposition temperature where the weight reduction begins ismeasured at a heating rate of 10° C./min using a thermogravimeter TGA-50available from Shimadzu Corporation.

(5) Melt Index (MI)

A KOUKA-SHIKI flow tester available from Shimadzu Corporation is used.Each polymer is put in a cylinder having an inner diameter of 9.5 mm,and after maintained at 230° C. for 5 minutes, is extruded by a 7 kgpiston into a piston weight portion, through an orifice having an innerdiameter of 2.1 mm and a length of 8 mm. The melt index is representedby a weight in gram of the copolymer extruded for 10 minutes.

(6) Light Transmission (T)

Polymethyl methacrylate for a core and the fluorine-containing copolymerfor a clad are spun into a composite fiber at 230° C. to make an opticalfiber having a diameter of 300 μm (thickness of clad 15 μm) and a lengthof 500 mm. A transmittance of this optical fiber is measured with lighthaving a wavelength of from 650 to 680 nm.

(7) Flexibility (F)

Measured by the method mentioned supra.

EXAMPLE 2

A fluorine-containing copolymer was prepared in the same manner as inExample 1 except that 60 parts of 6FNPM, 15 parts of MMA and 25 parts of8FM were used as monomers. The composition and properties of theobtained fluorine-containing copolymer were measured in the same manneras in Example 1. The results are shown in Table 1.

EXAMPLE 3

A fluorine-containing copolymer was prepared in the same manner as inExample 1 except that 45 parts of 6FNPM, 40 parts of MMA and 15 parts of8FM were used as monomers. The composition and properties of theobtained fluorine-containing copolymer were measured in the same manneras in Example 1. The results are shown in Table 1.

EXAMPLE 4

A fluorine-containing copolymer was prepared in the same manner as inExample 1 except that 58 parts of 6FNPM and 42 parts of MMA were used asmonomers. The composition and properties of the obtainedfluorine-containing copolymer were measured in the same manner as inExample 1. The results are shown in Table 1.

COMPARATIVE EXAMPLE 1

A homopolymer of 6FNPM was prepared in the same manner as in Example 1except that 100 parts of 6FNPM was used alone as a monomer. Thecomposition and properties of the obtained fluorine-containing polymerwere measured in the same manner as in Example 1. The results are shownin Table 1.

COMPARATIVE EXAMPLE 2

A fluorine-containing copolymer was prepared in the same manner as inExample 1 except that 90 parts of 6FNPM and 10 parts of MMA were used asmonomers. The composition and properties of the obtainedfluorine-containing copolymer were measured in the same manner as inExample 1. The results are shown in Table 1.

COMPARATIVE EXAMPLE 3

A fluorine-containing copolymer was prepared in the same manner as inExample 1 except that 50 parts of 6FNPM and 50 parts of MMA were used asmonomers. The composition and properties of the obtainedfluorine-containing copolymer were measured in the same manner as inExample 1. The results are shown in Table 1.

COMPARATIVE EXAMPLE 4

A fluorine-containing copolymer was prepared in the same manner as inExample 1 except that 30 parts of 6FNPM and 70 parts of MMA were used asmonomers. The composition and properties of the obtainedfluorine-containing copolymer were measured in the same manner as inExample 1. The results are shown in Table 1. TABLE 1 Example ComparativeExample 1 2 3 4 1 2 3 4 Monomers used (part by weight) 6FNPM 50 60 45 58100 90 50 30 MMA 30 15 40 42 — 10 50 70 8FM 20 25 15 — — — — — Copolymercomposition (% by mole) 6FNPM 34 49 28 34 100 77 27 14 MMA 54 33 64 66 —23 73 86 8FM 12 18 8 — — — — — Physical properties Mw 250,000 300,000280,000 400,000 220,000 240,000 310,000 370,000 F content (% by weight)32 34 27 24 43 39 22 13 Refractive index 1.425 1.421 1.434 1.437 1.3991.408 1.445 1.463 Tg (° C.) 105 110 120 120 123 120 121 119 Td (° C.)288 270 290 285 272 262 280 280 MI (g/10 min) 40 35 35 30 45 43 34 32Transmission (%) 84 82 85 82 80 81 80 75 Flexibility (radius mm) <6 <6<6 <6 20 20 15 <6

EXAMPLE 5

A fluorine-containing copolymer was prepared in the same manner as inExample 1 except that 45 parts of 6FNPM, 35 parts of MMA and 20 parts of4FM were used as monomers. The composition and properties of theobtained fluorine-containing copolymer were measured in the same manneras in Example 1. The results are shown in Table 2.

EXAMPLE 6

A fluorine-containing copolymer was prepared in the same manner as inExample 1 except that 50 parts of 6FNPM, 30 parts of MMA and 20 parts of8FF were used as monomers. The composition and properties of theobtained fluorine-containing copolymer were measured in the same manneras in Example 1. The results are shown in Table 2.

EXAMPLE 7

A fluorine-containing copolymer was prepared in the same manner as inExample 1 except that 50 parts of 6FNPF, 30 parts of MMA and 20 parts ofHFIPM were used as monomers. The composition and properties of theobtained fluorine-containing copolymer were measured in the same manneras in Example 1. The results are shown in Table 2.

EXAMPLE 8

A fluorine-containing copolymer was prepared in the same manner as inExample 1 except that 48 parts of 6FNPF, 35 parts of MMA and 17 parts of8FM were used as monomers. The composition and properties of theobtained fluorine-containing copolymer were measured in the same manneras in Example 1. The results are shown in Table 2.

EXAMPLE 9

A fluorine-containing copolymer was prepared in the same manner as inExample 1 except that 50 parts of 6FNPF, 30 parts of MMA and 20 parts of8FF were used as monomers. The composition and properties of theobtained fluorine-containing copolymer were measured in the same manneras in Example 1. The results are shown in Table 2. TABLE 2 Example 5 6 78 9 Monomers used (part by weight) 6FNPM 45 50 — — — 6FNPF — — 50 48 50MMA 35 30 30 35 30 4FM 20 — — — — 8FF — 20 — — 20 HFIPM — — 20 — — 8FM —— — 17 — Copolymer composition (% by mole) 6FNPM 27 34 — — — 6FNPF — —26 25 27 MMA 57 54 58 65 60 4FM 16 — — — — 8FF — 12 — — 13 HFIPM — — 16— — 8FM — — — 10 — Physical properties Mw 350,000 280,000 300,000250,000 220,000 F content 27 33 34 32 36 (% by weight) Refractive index1.435 1.422 1.411 1.416 1.408 Tg (° C.) 107 105 114 105 106 Td (° C.)280 295 300 290 310 MI (g/10 min) 32 36 36 43 44 Transmission (%) 85 8483 84 83 Flexibility (radius <6 <6 <6 <6 <6 mm)

INDUSTRIAL APPLICABILITY

The fluorine-containing optical materials according to the presentinvention assures a high Tg, a low refractive index, good flexibilityand low cost, which could not be attained in conventional opticalmaterials, and are very useful as materials for heat resistant plasticoptical fiber clad, particularly for plastic optical fiber for cars.

1. A fluorine-containing optical material which comprises afluorine-containing copolymer comprising from 32 to 36% by mole of astructural unit (a) represented by the formula (1):

wherein X¹ is H, CH₃, F, CF₃ or Cl; Rf² and Rf² are the same ordifferent and each is a perfluoroalkyl group having 1 to 5 carbon atoms;R¹ is a hydrocarbon group having 1 to 5 carbon atoms which may besubstituted with fluorine atom, and from 64 to 68% by mole of astructural unit (b) derived from methyl methacrylate.
 2. Afluorine-containing optical material which comprises afluorine-containing copolymer comprising from 15 to 62% by mole of astructural unit (a) represented by the formula (1):

wherein X¹ is H, CH₃, F, CF₃ or Cl; Rf¹ and Rf² are the same ordifferent and each is a perfluoroalkyl group having 1 to 5 carbon atoms;R¹ is a hydrocarbon group having 1 to 5 carbon atoms which may besubstituted with fluorine atom, from 12 to 70% by mole of a structuralunit (b) derived from methyl methacrylate and from 1 to 40% by mole of astructural unit (c) (excluding the structural unit (a)) derived from afluorine-containing monomer which is copolymerizable therewith.
 3. Thefluorine-containing optical material of claim 2, wherein in the formula(1), X¹ is CH₃.
 4. The fluorine-containing optical material of claim 3,wherein the fluorine-containing copolymer comprises from 23 to 50% bymole of the structural unit (a), from 33 to 70% by mole of thestructural unit (b) and from 1 to 40% by mole of the structural unit(c).
 5. The fluorine-containing optical material of claim 2, wherein inthe fluorine-containing copolymer, the structural unit (c) is astructural unit (c1) represented by the formula (2):

wherein X² is H, CH₃, F, CF₃ or Cl; R² is H or a fluoroalkyl group; thestructural unit represented by the formula (1) is excluded, and when R²is H, X² is neither H nor CH₃.
 6. The fluorine-containing opticalmaterial of claim 5, wherein in the formula (2), R² is a fluoroalkylgroup having 3 to 8 carbon atoms.
 7. The fluorine-containing opticalmaterial of claim 5, wherein the fluorine-containing copolymer comprisesfrom 23 to 50% by mole of the structural unit (a), from 33 to 70% bymole of the structural unit (b) and from 1 to 40% by mole of thestructural unit (c1).
 8. The fluorine-containing optical material ofclaim 5, wherein in the fluorine-containing copolymer, the number ofcarbon atoms of R² in the formula (2) representing the structural unit(c1) is from 4 to
 6. 9. The fluorine-containing optical material ofclaim 8, wherein in the fluorine-containing copolymer, R² in the formula(2) representing the structural unit (c1) is represented by the formula(3):—CH₂C_(n)F_(2n)H   (3) wherein n is an integer of from 3 to
 5. 10. Thefluorine-containing optical material of claim 8, wherein in thefluorine-containing copolymer, R² in the formula (2) representing thestructural unit (c1) is —CH₂C₄F₈H.
 11. The fluorine-containing opticalmaterial of claim 5, wherein in the fluorine-containing copolymer, X² inthe formula (2) representing the structural unit (c1) is —CH₃.
 12. Thefluorine-containing optical material of claim 1, which has a glasstransition temperature of not less than 100° C., a refractive index ofnot more than 1.440 and a fluorine content of not less than 20% byweight.
 13. The fluorine-containing optical material of claim 12,wherein the glass transition temperature is not less than 105° C. 14.The fluorine-containing optical material of claim 12, wherein therefractive index is not more than 1.430.
 15. The fluorine-containingoptical material of claim 12, wherein the fluorine content is not lessthan 30% by weight.
 16. A material for clad of optical fiber which isobtained from the fluorine-containing optical material of claim
 1. 17. Afluorine-containing copolymer which has a weight average molecularweight of from 10,000 to 1,000,000 and comprises from 32 to 36% by moleof a structural unit (a) represented by the formula (1):

wherein X¹ is H, CH₃, F, CF₃ or Cl; Rf¹ and Rf² are the same ordifferent and each is a perfluoroalkyl group having 1 to 5 carbon atoms;R¹ is a hydrocarbon group having 1 to 5 carbon atoms which may besubstituted with fluorine atom, and from 64 to 68% by mole of astructural unit (b) derived from methyl methacrylate.
 18. Thefluorine-containing copolymer of claim 17, wherein in the formula (1),X¹ is CH₃.
 19. A fluorine-containing copolymer which has a weightaverage molecular weight of from 10,000 to 1,000,000 and comprises from15 to 62% by mole of a structural unit (a) represented by the formula(1):

wherein X¹ is H, CH₃, F, CF₃ or Cl; Rf¹ and Rf² are the same ordifferent and each is a perfluoroalkyl group having 1 to 5 carbon atoms;R¹ is a hydrocarbon group having 1 to 5 carbon atoms which may besubstituted with fluorine atom, from 12 to 70% by mole of a structuralunit (b) derived from methyl methacrylate and from 1 to 40% by mole of astructural unit (c2) represented by the formula (2a):

wherein X³ is H, CH₃, F, CF₃ or Cl; R³ is H or a fluoroalkyl group; thestructural unit represented by the formula (1) is excluded, and when R³is H, X³ is neither H nor CH₃.
 20. The fluorine-containing copolymer ofclaim 19, wherein in the formula (1), X¹ is CH₃.
 21. Thefluorine-containing copolymer of claim 19, which comprises from 23 to50% by mole of the structural unit (a), from 33 to 70% by mole of thestructural unit (b) and from 1 to 40% by mole of the structural unit(c2).
 22. The fluorine-containing copolymer of claim 19, wherein thenumber of carbon atoms of R³ in the formula (2a) representing thestructural unit (c2) is from 4 to
 6. 23. The fluorine-containingcopolymer of claim 22, wherein R³ in the formula (2a) representing thestructural unit (c2) is represented by the formula (3):—CH₂C_(n)F_(2n)H   (3) wherein n is an integer of from 3 to
 5. 24. Thefluorine-containing copolymer of claim 22, wherein R³ in the formula(2a) representing the structural unit (c2) is —CH₂C₄F₈H.
 25. Thefluorine-containing copolymer of claim 19, wherein X³ in the formula(2a) representing the structural unit (c2) is —CH₃.
 26. Thefluorine-containing optical material of claim 1, wherein in the formula(1), X¹ is CH₃.
 27. The fluorine-containing optical material of claim 2,which has a glass transition temperature of not less than 100° C., arefractive index of not more than 1.440 and a fluorine content of notless than 20% by weight.
 28. The fluorine-containing optical material ofclaim 27, wherein the glass transition temperature is not less than 105°C.
 29. The fluorine-containing optical material of claim 27, wherein therefractive index is not more than 1.430.
 30. The fluorine-containingoptical material of claim 27, wherein the fluorine content is not lessthan 30% by weight.
 31. A material for clad of optical fiber which isobtained from the fluorine-containing optical material of claim 2.